The association of circular lines-of-position with horizontal bearing differences is a technique well known in navigation (see, for example, G. P. Clark, "Simplified Determination of the Ellipse of Uncertainty", Navigation: Journal of the Institute of Navigation, Vol. 21, No. 4, 1974). Royal, in U.S. Pat. No. 3,922,533 describes its use in multi-platform RF emitter location. In U.S. Pat. No. 5,526,001, the instant inventor presented a method to generate the circles employing phase difference measurements from a two antenna, uncalibrated, long-baseline-interferometer (LBI). When deriving the bearing difference from LBI phase differences, measurement precision is proportional to the interferometer baseline length, and inversely proportional to the emitter's frequency. Since the circular lines-of-position are generated from LBI phase measurements they are called phase circles.
FIG. 1 illustrates the unambiguous phase circles generated for arbitrary emitter locations relative to the observer. Because the LBI baseline d 108 (FIG. 1) is typically hundreds of wavelengths long, the phase measurements 109 give extremely high spatial angle resolution 103, but are highly ambiguous. The phase difference 107 measured between points 111 m.sub.1 and 112 m.sub.2 is associated with the phase circle 100 through the angle change measurement 106. But a consequence of the ambiguities 101 on the individual phase measurements is that the angle difference 106 is also ambiguous 105, and hence a family of phases circles 102 results. Each member of this family passes through the fixed points m.sub.1 and m.sub.2 marking the beginning and end of the phase change measurement. The track 104 the observer flies between these two points is arbitrary, and does not directly affect the generation of the family of phase circles. To obtain the emitter location a second set of phase circles must intercept this first family, and also the phase ambiguities must be correctly resolved. A second observer typically generates the second set of phase circles.
Note that FIG. 1 alternatively illustrates, for unambiguous phase measurements, the phase circles generated for arbitrary emitter locations relative to the observer. That is, for all specific ambiguity integers possible for all emitter locations, it illustrates the resulting phase circles through the emitters. Both interpretations of the figure are important for understanding the improvements introduced by the present invention because ambiguous phase circle interpretation is germane to one of the key features of the invention: optimal performance independent of the eventual ambiguity resolution process, i.e., independent of the actual values of 105 N.sub.1 -N.sub.2 corresponding to the emitter position 113.
In generating the second family of possible COP, the only information available is the start position (111FIG. 1) and predicted end point 112 of the observer making the initial; emitter detection.